1. Field of the Invention
The present invention relates to a control apparatus of a spark ignition engine and, more particularly, to a control apparatus and a control method suitable for the combustion stabilization of a spark ignition engine that controls the intake air flow by an intake valve and an exhaust valve, which are provided with mainly a movable valve mechanism.
2. Background Art
As measures to improve the fuel consumption of a spark ignition internal combustion engine used in automobiles and the like, a reduction of pumping losses of the engine is effective. In recent years, attention has been given to engines that aim to reduce pump losses by providing a variable valve mechanism in an intake valve and by controlling the intake air flow not by use of a throttle provided in the intake valve, but mainly by the opening and closing timing of the intake valve (early closing of the intake valve).
In these engines, however, compared to usual operation performed through the use of a throttle, the compression end temperature decreases because the effective compression ratio decreases as a result of early closing of the intake valve, and hence combustion tends to become unstable particularly during low-load operation.
As measures to avoid unstable combustion during low-load operation, first, there are known engines in which the intake air flow is controlled by using a throttle instead of an intake valve only under extremely low load, such as idling, whereby the effective compression ratio is improved and unstable combustion is avoided (refer to JP Patent Publication (Kokai) No. 2001-159329 A, for example).
Second, there are known engines in which combustion stability is improved by improving the effective compression ratio by retarding the opening and closing timing of an intake valve in angle under the low-load conditions of idling and cooling fan operation and an increase in the intake air flow caused by this retarding in angle is suppressed by reducing the lift amount of the intake valve (refer to JP Patent Publication (Kokai) No. 2006-336494 A, for example).